This invention relates generally to the field of marine propulsion, and more particularly to an electrical generator for a marine engine, and specifically to a device and method for attaching magnets to the flywheel of a marine engine electrical generator.
It is known to generate electricity by moving a permanent magnet past a stator coil. In the field of marine engines, it is known to attach a plurality of permanent magnets to an inside diameter surface of the flywheel of an engine, and to install a stator proximate the inside diameter surface in order to induce an electrical current in the stator as the magnets are rotated past the stator by the revolution of the flywheel. Such magnets may be attached to the flywheel in a variety of ways. For example, it is known to glue magnets to a flywheel. An adhesive such as an epoxy glue is applied to a magnet and/or flywheel, and the magnet is pressed against the surface of the flywheel while the adhesive hardens. This method leaves the magnet exposed and subject to damage during handling of the flywheel. In addition, corrosion behind the magnets may reduce the bond strength. Alternatively, a magnet may be retained against a flywheel by a metal retaining plate, which in turn, is attached to the flywheel with fasteners. While the retaining plate serves as a protective shield for the magnets, the additional thickness of the plate increases the gap between the magnets and the stator, thereby decreasing the efficiency of the electrical generator.
It is also known to mold a cage around magnets attached to an inside diameter surface of a flywheel. Such a cage forms a protective ring of plastic, fiberglass reinforced nlyon, or other molding material to completely enclose the magnets. A thin metal shim stock is positioned around the inside surface of the flywheel to define the volume of the cage, and liquid molding material is introduced into the volume to surround the magnets. Once hardened, the molding material forms a protective ring encircling the magnets. Similar to the metal retaining plate design described above, however, this design disposes a metal shim stock between the magnets and the stator. Furthermore, the flexibility of the shim stock may cause the inside diameter of the protective ring to be uneven, thereby necessitating an undesirable increase in the design gap between the magnets and the stator.
What is needed is a method and apparatus for retaining magnets on a flywheel that provides protection for and mechanical retention of the magnets without necessitating the need for an increase in the gap between the magnets and the stator.
An improved flywheel is disclosed herein having: a disk portion; a rim portion attached to the disk portion and having an inside diameter surface; a depression formed in the surface of at least one of the disk portion and the rim portion; a magnet attached to the inside diameter surface; and a cage in contact with a magnet and extending into the depression. The flywheel is further described as having an injection molding shutoff surface formed on at least one of the disk portion and the rim portion of the flywheel proximate an edge of the cage. The injection molding shutoff surface is formed to mate with a male portion of a die during an injection molding process. The depression formed in the flywheel may be a groove formed in the rim portion or the disk portion. The portion of the cage extending into the groove(s) resists the shrinkage of the cage as it cools following the injection molding process, thereby assuring precise control of the cage inside diameter. To further minimize the gap between the magnets and the stator of an electrical generator incorporating this invention, the magnets may be attached to the inside diameter surface of the flywheel by a process that includes the steps of: forming a smooth inside diameter surface on the flywheel; forming a smooth inside diameter surface and opposed smooth outside diameter surface on the magnet; applying adhesive to at least on the magnet outside diameter surface and the flywheel inside diameter surface; pressing the magnet outside diameter surface against the flywheel inside diameter surface with the adhesive therebetween; and holding the magnet inside diameter surface at a predetermined diametrical position while the adhesive hardens.